This invention relates to a theftproof device which includes an optical fiber extending through a flexible tying member, such as a wire cable, and operable to photoelectrically detect cutting of the flexible tying member, for giving warning.
A theftproof device for vehicles such as a motorcycle is conventionally used, which is of the so-called wire and lock type using a wire cable as a tying member. According to this conventional device, a vehicle, which is to be protected from theft, is tied to a solid structure, such as an electric pole, by means of the wire cable, in such a manner that two lock elements secured on the opposite ends of the wire cable are locked together.
However, vehicle thefts have recently increased in number due to diffusion of improved cutting tools, such as wire cutters, since the wire cable of the theftproof device can be easily cut by such improved cutting tools. To prevent this, a theftproof device has also been proposed and is actually used, which includes electric conductors or wires disposed through the wire cable, in which electric current permanently flows. The device is adapted to detect cutting of the electric conductors together with the wire cable, for giving warning.
However, this electric type device cannot avoid forming a roundabout circuit for electric current to flow therethrough, which can supersede the original electric cicuit formed in the wire cable to render same inoperative. As a consequence, there still occur a considerable number of vehicle thefts, wherein the wire cable is cut after the electric circuit therein has been rendered inoperative in the above manner.